Polymers each of which contains, as a main component, an isoolefin monomer typified by isobutylene are widely used for various sealing members due to their high gas barrier property. Known examples of such polymers include a styrene-b-isobutylene-b-styrene block copolymer (SIBS), an isobutylene/isoprene polymer, an isobutylene/isoprene polymer halogenated by chlorine or bromine, and an isobutylene/p-methyl styrene polymer halogenated by chlorine or bromine.
SIBS consists of (i) a low Tg block that is a middle block and is constituted by an isobutylene polymer and (ii) a high Tg block that is a both end block and is constituted by a styrene polymer. Such a structure causes a thermoplastic elastomer to display a characteristic of having rubber elasticity at a room temperature and being liquefied at a high temperature of not less than 100° C. The SIBS is known to display, especially at the room temperature, an excellent dynamic property of having both plasticity and a strength.
Patent Literature 1 discloses a method for producing a block copolymer including (i) a block that contains isobutylene as a main component and (ii) a block that contains an aromatic vinyl monomer as a main component. Patent Literature 1 further discloses that the block that contains isobutylene as a main component may contain, in an amount of not more than 40% by weight, other monomers that are cationically polymerizable. Note, however, that Patent Literature 1 discloses no method for causing the block copolymer to maintain a dynamic property such as a tensile strength also in a case where those monomers are copolymerized.
A brominated isobutylene/p-methyl styrene copolymer, which is one of halogenated isobutylene polymers, is produced by obtaining an isobutylene/p-methyl styrene copolymer by cationic polymerization and then causing a light or radical generator to act on the isobutylene/p-methyl styrene copolymer in the presence of halogen so as to halogenate a p-methyl group (Patent Literature 2). Among isoolefin resins, a halogenated isobutylene polymer is used for various applications. This is because the halogenated isobutylene polymer has a halogen group, which is a functional group, while maintaining its high gas barrier property. For example, it can be expected that introduction of a polar group such as a halogen group into an isoolefin polymer having a low polarity dramatically improves adhesion, to another composition, of the isoolefin polymer into which the halogen group has been introduced. Further, since a halogenated p-halogenated methyl group can easily substitutively react with a nucleophilic reagent in an aprotic polar solvent, various functional groups other than a halogen group can be introduced into the isoolefin polymer. For example, Patent Literature 3 reports that an acryloyl group, which is a functional group having a radical reactivity, is introduced into a halogenated isobutylene/p-methyl styrene copolymer by causing an acrylic compound to act on the halogenated isobutylene/p-methyl styrene copolymer.
Patent Literatures 2 and 4 each disclose a method for producing an isoolefin/p-alkylstyrene copolymer, which is a precursor to such an isoolefin polymer as described above. Note, however, that a molecular weight distribution of the produced copolymer is as wide as approximately 2 according to Patent Literatures 2 and 4 and neither of Patent Literatures 2 and 4 discloses a polymerization method for allowing maintenance of an active terminal that is capable of continuous propagation.
Such a thermoplastic elastomer as described above, the thermoplastic elastomer having a functional group such as a halogen group or an acryloyl group, has adhesion and reactivity and thus is frequently used by being compounded with another material. In this case, the thermoplastic elastomer is required to have shape retentivity at a plasticization temperature or a curing temperature of the another material, i.e., heat resistance. For example, in a case where a composite material of (i) a thermoplastic elastomer having a UV-curable acryloyl group and (ii) vulcanized rubber is assumed and the thermoplastic elastomer can also maintain its shape at approximately 200° C. at which to vulcanize the rubber, variations of a method for molding the composite material are increased.
Patent Literature 5 discloses a method for producing a triblock copolymer including an alkylstyrene/isoolefin polymer block and an aromatic vinyl polymer block, the triblock copolymer being produced by initiating copolymerization of isoolefin and alkylstyrene which are mixed in advance, adding isoolefin at a midpoint of the polymerization at which midpoint those monomers have been consumed to some extent, and further adding an aromatic vinyl monomer. Note, however, that Patent Literature 5 reports neither a concentration nor an effect of alkylstyrene.
Further, Patent Literature 5 discloses an example of production of a copolymer in which p-methyl styrene is separately added several times during polymerization of isobutylene so as to be randomly introduced into an isobutylene polymer and which has a narrow molecular weight distribution. Note, however, that Patent Literature 5 does not report that the triblock copolymer is produced by polymerizing the aromatic vinyl monomer subsequently to the introduction of p-methyl styrene into the isobutylene polymer.